The present invention relates to a semiconductor device, for example, to a circuit for improving the output characteristics of an output stage of a semiconductor device, Particularly, the present invention relates to a semiconductor device having a semiconductor element referred to as an open drain, an open collector or the like which becomes operable when connected to an external power source.
An open drain is conventionally known and used as an output stage of the above type of semiconductor device. For example, as shown in FIG. 11, such a semiconductor device has a structure in which the source of an N-type MOS transistor 10 constituting the output stage is connected to the ground, and the drain thereof in connected to a pad 20 which is an electrode used for external connection. In the semiconductor device having such a structure, a second power source potential VDD2 independent of a first power source potential VDD1 to be supplied to the inside of the semiconductor device is applied to the pad 20 via a pull-up resistor 30. A signal from a circuit (not shown) preceding a buffer circuit 40 is applied to the gate of this N-type MOS transistor 10 via the buffer circuit 40. The buffer circuit 40 operates by the supply of the first power source potential VDD1.
However, the following problem exists in the conventional devices having the above structure.
Specifically, when the first power source potential VDD1 to be supplied to the inside of the semiconductor device is shut off, the buffer circuit 40 becomes non-operational. For this reason, normally, the potential of an output line 42 of the buffer circuit 40 connected to the gate of the N-type MOS transistor 10 should become zero to make the MOS transistor 10 nonconductive. The potential of the pad 20 would be thereby held at the second power source potential VDD2. However, the potential of the output line 42 does not become zero in actual operation and the MOS transistor 10 becomes incompletely nonconductive. This brings about the problem that the potential of the pad 20 is not maintained at the second power source potential VDD2.
For example, if the potential of the pad 20 is not raised to the second power source potential VDD2 and another semiconductor chip is connected through the pad 20, the current may flow from the circuits of other semiconductor chips into the MOS transistor 10.
The inventor has directed his attention to fact that the above problem is caused because the potential of the output line 42 is not lowered sufficiently to make the MOS transistor 10 completely nonconductive due to parasitic capacitance and the like in a semiconductor substrate even if the supply of the first power source potential VDD1 is shut off, and achieved the present invention.
An objective of the present invention is to provide a semiconductor device which can reliably switch the operating state of a transistor to which a second power source potential is supplied according to the state of supply of a first power sources potential.
Another objective of the present invention is to provide a semiconductor device which can reliably switch the operating state of a transistor which is used as an open drain or open collector.
A further objective of the present invention is to provide a semiconductor device which can improve the operational reliability of an output stage thereof.
According to an aspect of the present invention, there is provided a semiconductor device according comprising:
a buffer circuit which is operated by supply of a first power source potential;
a transistor which is provided on a supply line of a second power source potential and has a gate connected to an output line of the buffer circuit; and
a potential setting circuit which sets a potential of the output line of the buffer circuit to a level lower than the threshold level of the transistor, when the supply of the first power source potential is shut off.
In this aspect of the invention, when the supply of the first power source potential is shut off, the potential of the output line of the buffer, that is, the potential between the gate and source of the transistor is set to a level lower than the threshold level of the transistor. As a result of this, the transistor can be reliably made non-operational. Therefore, in contrast to the conventional art, the occurrence of such an event that the transistor becomes nonconductive only incompletely although the supply of the first power source potential in shut off can be reliably avoided.
The transistor may be an N-type MOS transistor. As an example of a case using an N-type MOS transistor, there is a case where the semiconductor device comprises a pull-up resistor and a pad to which the second power source potential is supplied through the pull-up resistor. In this case, the N-type MOS transistor is connected between the pad and a ground. In the case using the N-type MOSFET, the potential setting circuit may preferably set the potential of the output line of the buffer circuit to the ground potential when the supply of the first power source potential in shut off. This makes it possible to make the N-type MOS transistor completly nonconductive.
Preferred examples of the potential setting circuit will be described below.
The potential setting circuit may have a firs N-type MOS transistor and a second N-type MOSTET which are connected in parallel between the output line of the buffer circuit and the ground. The first N-type MOS transistor has a gate connected to the output line of the buffer circuit, and the second N-type transistor has a gate connected to the pad.
In this example, the first N-type MOS transistor functions as a diode forward-connected between the output line of the buffer circuit and the ground. Therefore, the potential of the output line of the buffer circuit can be reliably dropped to the threshold level of the first N-type MOSFET by the first N-type MOS transistor when the supply of the first power source potential is shut off. If the potential of the output line of the buffer circuit is designed to be equal to or less than the threshold level of the first N-type MOSFET, the N-type MOS transistor can be made nonconductive. And then if the potential of the pad is increased to a level equal to or higher than the threshold level of the second N-type MOS transistor, the second N-type MOS transistor becomes conductive. As a result, the potential of the output line of the buffer can be dropped to the ground potential.
In another example, the potential setting circuit may have a P-type MOS transistor connected between the output line of the buffer circuit and the ground The first power source potential may be supplied to a gate of the P-type MOS transistor. In this case, when the supply of the first power source potential is shut off, the P-type MOS transistor becomes conductive and the potential of the output line of the buffer circuit can be reliably dropped to the threshold level of the P-type MOS transistor. If the potential of the output line of the buffer circuit is made to a level equal to or less than the threshold level of the N-type MOS transistor, the N-type MOS transistor can be made nonconductive.
In a further example, the potential setting circuit may have a first N-type MOS transistor connected between the output line of the buffer circuit and the ground, and a potential applying circuit which applies a potential sufficient to take the first N-type MOS transistor conductive to a gate of the first N-type MOS transistor, when the supply of the first power source potential is shut off. The potential applying circuit may have: a second N-type MOSFET, a P-type MOS transistor, and a third N-type MOS transistor which are connected in series between a supply line of the first power source potential and the ground; and a capacitance connected between the ground and wiring for connecting the second N-type MOS transistor to the P-type MOS transistor. In this case, gates of the second N-type MOS transistor, P-type MOS transistor, and third N-type MOS transistor are connected to the supply line of the first power source potential.
In this example, electric charges are supplied to the capacitance when the first power source potential is supplied. When the supply of the first power source potential is shut off, a potential based on the charge supplied to the capacitance is applied to the gate of the first N-type MOS transistor and the first N-type MOS transistor is made conductive. Therefore, the potential of the output line of the buffer circuit is reliably change to the ground potential.
The above transistor may be a P-type MOS transistor instead of the N-type MOS transistor. Examples of a case using the P-type MOS transistor may include a case where the semiconductor device has a pull-down resistor and a pad to which the second power source potential is supplied through the pull-down resistor. In this case, the P-type MOS transistor is connected between the pad and a supply line of a third power source potential which in higher than the first power source potential.
When a P-type MOS transistor is used as the transistor, a similar structural example to the above various structural examples of the potential setting circuit for setting the gate potential of the N-type MOS transistor may be used. In this case, among the semiconductor elements constituting the potential setting circuit, the P-type must be replaced by N-types and the N-types must be replaced by P-types.
According to another aspect of the present invention, there in provided a semiconductor device comprising:
a buffer circuit which is operated by supply of a first power source potential;
a pull-up resistor;
a pad to which a second power source potential is supplied through the pull-up resistor;
a first N-type MOS transistor which is connected between the pad end a ground and has a gate connected to an output line of the buffer circuit; and
a second N-type MOS transistor which is connected between the pad and the first N-type MOS transistor and has a gate connected to a supply line of the first power source potential.
In this aspect of the invention, when the supply of the first power source potential is shut off, the second N-type MOS transistor becomes nonconductive, and the potential of the pad can be made the second power source potential, regardless of the operational state of the first N-type MOS transistor.
According to a further aspect of the present invention, there is provided a semiconductor device comprising:
a buffer circuit which is operated by supply of a first power source potential;
a pull-down resistor;
a pad to which a second power source potential is supplied through the pull-down resistor;
a first P-type MOS transistor which is connected between the pad and a supply line of a third power source potential which is higher than the first power source potential, the first P-type MOS transistor having a gate connected to an output line of the buffer circuit; and
a second N-type MOS transistor which in connected between the pad and the first P-type MOS transistor and has a gate connected to a supply line of the first power source potential.
In this aspect of the invention, when the supply of the first power source potential is shut off, the second P-type MOS transistor becomes nonconductive and the potential of the pad can be made the second power source potential, regardless of the operational state of the first P-type MOS transistor.
In this way, according to these aspects of the present invention, the potential between the gate and source of the output transistor can be kept at a level sufficient to make the output transistor completely non-operational when the power source of the buffer circuit in a stage preceding the output transistor as an open drain is shut off. For this reason, in contrast to the conventional art, the occurrence of such an event that the output transistor becomes incompletely non-operational and the potential of the drain does not reach a predetermined potential can be avoided. Accordingly, the operation of the output transistor can be stably and reliably switched, thereby improving the reliability of the circuit operations.
Also, since addition of several semiconductor elements in addition to the output transistor is sufficient, the operation of the output stage can be ensured by a simple structure.